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Article Cite This: ACS Omega 2018, 3, 1581−1588
Facile Synthesis of SnS2 Nanostructures with Different Morphologies for High-Performance Supercapacitor Applications Nazish Parveen,† Sajid Ali Ansari,*,†,‡ Hatem R. Alamri,§ Mohammad Omaish Ansari,∥ Ziyauddin Khan,⊥,# and Moo Hwan Cho*,† †
School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 712-749, South Korea Department of Energy & Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea § Physics Department, Jamoum University College, Umm Al-Qura University, Makkah 21955, Saudi Arabia ∥ Center of Nanotechnology, King Abdulaziz University, Jeddah 21589, Saudi Arabia ⊥ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea # Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, SE-601 74 Norrköping, Sweden ‡
S Supporting Information *
ABSTRACT: SnS2 is an emerging candidate for an electrode material because of the considerable interlayer spaces in its crystal structures and the large surface area. SnS2 as a photocatalyst and in lithium ion batteries has been reported. On the other hand, there are only a few reports of their supercapacitor applications. In this study, sheetlike SnS2 (SL-SnS2), flowerlike SnS2 (FL-SnS2), and ellipsoid-like SnS2 (EL-SnS2) were fabricated via a facile solvothermal route using different types of solvents. The results suggested that the FL-SnS2 exhibited better capacitive performance than the SL-SnS2 and EL-SnS2, which means that the morphology has a significant effect on the electrochemical reaction. The FL-SnS2 displayed higher supercapacitor performance with a high capacity of approximately ∼431.82 F/g at a current density of 1 A/g. The remarkable electrochemical performance of the FL-SnS2 could be attributed to the large specific surface area and better average pore size. These results suggest that a suitable solvent is appropriate for the large-scale construction of SnS2 with different morphologies and also has huge potential in the practical applications of high-performance supercapacitors.
1. INTRODUCTION The urgent demand for high energy density and power density and supportable energy sources has prompted research into novel storage devices. At the same time, the increasing demand for portable devices has increased the urgency to find storage devices that are capable and safe for storing energy. Therefore, the emergence of supercapacitors has changed the situation to some degree. Because supercapacitors or electrochemical supercapacitors have properties, such as rapid charging/ discharging time, high power density, and long life, these types of energy devices are used widely.1−7 Electrochemical supercapacitors can be classified as electric double-layer capacitors (EDLC) or pseudocapacitors based on their energy storage mechanism. Carbon materials work based on the double-layer mechanism and store energy by accumulating charge at the interface between the electrode and the electrolyte.8 On the other hand, metal oxides/hydroxide materials work mainly on the faradic mechanism, storing energy by reversible oxidation and reduction reactions on the electrode material surface.9 In recent years, the electrodes of supercapacitors have not been limited to carbon-based © 2018 American Chemical Society
materials, transition-metal oxides, conducting polymers, and hydroxides. Transition-metal sulfides, which are a fascinating class of electrode material, such as tin sulfide, tin disulfide, tungsten disulfide, copper sulfide, and nickel sulfide, have been studied extensively for potential energy storage applications.9−11 Zhang et al. described a simple in situ growth procedure that produced a three-dimensional (3D) interconnected copper sulfide nanowall, which showed a high performance at a current density of 15 mA/cm2.12 The improved electrochemical performance may be dependent on the high conductivity and large surface area. Li et al. reported the synthesis of grasslike Ni3S2 nanorod/nanowire arrays by precisely regulating the degree of oxidation of the NF precursor, achieving a high specific capacitance of 4.52 F/cm2 at 1.25 mA/cm2 and showing superior cycling stability.13 Chauhan et al. reported that the tin sulfide nanorod exhibited a specific capacitance of approximately 70 F/g.14 This showed Received: December 11, 2017 Accepted: January 24, 2018 Published: February 7, 2018 1581
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Figure 1. SEM images of SnS2 with different solvents: (a−c) FL-SnS2, (d−f) SL-SnS2, and (g−i) EL-SnS2.
SnS2 exhibited better electrochemical performance because of these advantages.
that the specific surface area is a critical parameter to determine the specific capacitance of these materials.15 The results show that the special morphological structure of the material is a remarkable advantage. Moreover, SnS2 has been reported to have outstanding properties for applications in photocatalysis, electrochemical capacitors and batteries, but its applications in supercapacitors have been limited by its structural instability and lower specific capacitance.16,17 Only a few results have been reported for supercapacitor materials based on SnS2 electrodes.18 Ma et al. reported a molybdenum-doped few-layered SnS2 morphology synthesized by a hydrothermal method and established the improved electrochemical performance with a high specific capacitance of ∼220 F/g and good cycling stability.16 Wang et al. examined the role of SnS2/MoS2 heterostructures in enhancing the supercapacitive performance and reported a specific capacitance of ∼105.7 F/g at a current density of ∼2.350 A/g with good cycling stability after 1000 cycles (∼90.40%). These studies included composite materials and hybrid materials of SnS2 rather than pure SnS2. In the present case, different nanostructured SnS2 materials were synthesized with a better pore size and high surface area, showing better capacitance than the hybrid materials reported elsewhere.11,14,18 SnS2 was synthesized with different morphologies using a one-step solvothermal method, and its electrochemical capacitor performance was investigated. The nanosheetassembled flowerlike SnS2 (FL-SnS2) reached approximately ∼431.82 F/g at a current load of 1 A/g, which is higher than those of sheetlike SnS2 (SL-SnS2) (∼390.38 F/g) and ellipsoidlike SnS2 (EL-SnS2) (∼117.72 F/g). Because of the unique morphological structure of the nanosheet-assembled FL-SnS2, the surface area of the FL-SnS2 (∼64.8 m2/g) was higher than those of the SL-SnS2 (∼27.4 m2/g) and EL-SnS2 (∼32.9 m2/g), which exhibited remarkable advantages, including rich available electroactive sites and sufficient transmission channels. The FL-
2. RESULTS AND DISCUSSION Nanostructured materials, having attractive and interesting characteristics, can be synthesized using hydrothermal or solvothermal methods. In the present case, tin chloride was treated solvothermally with thioacetamide (TAA) and the resulting powdered morphology was analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). Figures 1 and S1 show the low-magnification SEM images indicating the formation of FL-SnS2 with highly uniform and homogeneous microspheres, ∼2−4 μm in diameter (Figure 1a,b). On the other hand, the high-magnification SEM image showed that each SnS2 is made by the self-assembly of ultrathin vertically grown connected nanopetals, which leads to the formation of a hierarchical 3D SnS2 nanostructure (Figures 1c and S1a). The SnS2 synthesized using acetone as a solvent had a twodimensional (2D) sheetlike morphology, which was ∼100−200 nm in width and ∼20−30 nm in thickness (Figures 1d−f and S1b). In contrast, the SnS2 synthesized using water as a solvent showed irregular ellipsoidal shapes of a SnS2 nanostructure with diameters ranging from ∼10 to ∼15 nm (Figures 1g−i and S1c). TEM also confirmed the formation of vertically grown connected nanopetals (Figures 2a,b and S2a), which supports the SEM results. Each individual flower consisted of a large number of nanosheets. HRTEM confirmed that the SnS2 nanopetals were crystalline in nature and grew in the direction of the (001) plane (Figure 2b) with a lattice spacing of ∼0.59 nm. TEM of the SL-SnS2 and EL-SnS2 further confirmed the structural features of the well-defined hexagonal plate and eclipsed (Figures 2c,e and S2b,c), which was also verified by HRTEM (Figure 2d,f). This suggests that the surface of the 1582
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Figure 3. Schematic diagram of the formation mechanism of SnS2 nanostructures.
Sn 4 + + S2 − → SnS2 ( ↓ )
This mechanism can be explained in two ways. The first is the precipitation of the metal sulfide by TAA at a low pH, and the second is the direct reaction of TAA with the metal precursor at a high pH.21,22 The presence of acetic acid in the present case obviously follows the first condition, in which the hydrolysis of TAA formed hydrogen sulfide (H2S), which is followed by the formation of small SnS2 nuclei. These tiny nuclei grow and form the 2D nanostructures because of the anisotropic crystal structure of SnS2.23 The presence of the H2S gas bubbles in the solution due to the hydrolysis of TAA induces the aggregation and self-assembly of the 2D SnS2 onto the gas bubbles at the gas−liquid interface to minimize the interfacial energy.24 These processes ultimately lead to the formation of 3D SnS2 during the optimized reaction condition, such as temperature and time. On the other hand, the lower reaction time may reduce the aggregation of tiny 2D crystals of the SnS2, which leads to the formation of immature 3D SnS2, whereas no significant change was observed at prolonged reaction times up to 12 h. In addition, acetone and water were also used for further investigation of the solvent effect, and different morphologies were observed in the cases of acetone and water. Only the ellipsoid-shaped-type morphology was observed in the case of water, which clearly highlights the role of the solvent in controlling the morphology of the SnS2. Prior to crystal growth and assembly, the tin anion forms a complex with acetic acid while the decomposition of TAA leads to the formation of H2S, which is driven by the ramping temperature of the electric oven. The H2S formed dissociates further into hydrogen and sulfide ions. The presence of tin ions in the solution induces the formation of a SnS2 colloid. When MeOH was used, the initial SnS2 nanosheets and SnS2 tiny crystal seeds formed progressively, and during the growth process, they tended to self-assemble and aggregate to form 3D SnS2 nanopetals. On the other hand, various process, such as nucleation, growth aggregation, self-assembly, and Ostwald ripening, may also be involved in the growth of SnS2. Although the precise mechanism for the growth of the nanostructures is unclear and requires a detailed investigation, these 3D assembly structures have been reported elsewhere.25,26 A separate synthesis experiment with different solvents, such as acetone and water, was carried out to confirm the role of the solvent in the formation of the morphology. A 2D sheetlike morphology was obtained when acetone was used instead of ethanol (Figures 1d−f and S1b). In the case of water as a solvent,
Figure 2. TEM and HRTEM images of the SnS2 nanostructures with different solvents: (a,b) FL-SnS2, (c,d) SL-SnS2, and (e,f) EL-SnS2.
nanosheet is composed mainly of {001} facets, which is in good agreement with the X-ray diffraction (XRD) patterns. Energydispersive X-ray (EDX) analysis was conducted to confirm the presence of sulfur (S) and tin (Sn) in the sample, as shown in Figure S3. On the basis of the SEM and TEM morphological structures, the formation of nanostructures can be described by considering the various steps involved. The solvents and precursors play important roles in catalyzing the reaction and increasing the kinetics of the solvothermal reaction. Previous reports recommend that the concentration of the sulfur source also plays a critical role in the growth and assembly of SnS2 nanostructures.20 Figure 3 presents a schematic diagram of the growth of the 2D formation of SnS2 sheets, 3D FL-SnS2 nanopetals, and ellipsoid-shaped SnS2 structures via the solvothermal process. The 3D SnS2 flowers were formed from the assembly of twodirectional sheets of SnS2, which was constructed using various steps, including nucleation, growth aggregation, self-assembly, and Ostwald ripening. On the other hand, the precise mechanism of the formation of such architectures is still unclear and requires a detailed investigation. On the basis of previous reports and the present observation and optimization, the sulfur concentration and different solvents play an important role in constructing such types of architectures of SnS2. In the beginning of the solvothermal reaction, tin anions interact with acetic acid and form a complex, whereas TAA decomposition leads to the formation of H2S at elevated temperatures, which can be illustrated in the following reaction: CH3CSNH 2 + H 2O → CH3CONH 2 + H 2S
(1)
H 2S → 2H+ + S2 −
(2)
(3)
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Figure 4. (a) XRD patterns; (b) N2 adsorption−desorption isotherm; (c) pore size distribution calculated from desorption branch spectra; (d) X-ray photoelectron spectroscopy (XPS) survey spectra of SL-SnS2, EL-SnS2, and FL-SnS2; (e) high-resolution XPS spectra of Sn 3d; and (f) S 2p elements present in SnS2.
Figure 5. (a) Comparative cyclic voltammograms at 50 mV/s, (b) comparative CD profiles of the EL-SnS2, FL-SnS2, and SL-SnS2 electrodes at a fixed current load of 1 A/g, (c) CD profile of the FL-SnS2 at different current loads, (d) CD profile of the EL-SnS2 at different current loads, (e) CD profile of the SL-SnS2 at different current loads, and (f) specific capacitance of the EL-SnS2, FL-SnS2, and SL-SnS2 at different current densities.
processing conditions influence the oriented growth of the SnS2 products, which is in keeping with the SEM and TEM results. For energy storage applications, the texture properties of the materials play a crucial role by providing sufficient surface area to store the charge during the electrochemical measurement.23 Therefore, a careful investigation of the qualitative and quantitative properties of SnS2 nanostructures was conducted by the N2 adsorption−desorption analysis carried out at 77 K. Table S2 lists the mean pore size, pore volume, and Brunauer− Emmett−Teller surface area of the SnS2 nanostructures. The surface area of the SnS2 samples is affected by the solvents. As shown in Figure 4b, the FL-SnS2 sample exhibited the characteristic type IV adsorption isotherms with a sharp adsorption at low pressures and a prominent hysteresis loop at the intermediate pressure range, indicating that the FL-SnS2 possesses a mesoporous structure. Among the SnS2 nanostruc-
however, the morphology was completely different (Figures 1g−i and S1c). These results suggest that the solvent plays a critical role in the growth and assembly of SnS2 nanocrystals. The crystal and phase structures of the as-prepared SnS2 nanostructures were examined by XRD, as shown in Figure 4a. All phases of the SnS2 nanostructures were well-matched with the standard plane position of JCPDS 022-0951 with lattice parameters of a = b = 3.648 Å and c = 5.894 Å of the SnS2 nanostructures. According to the Bragg equation, the calculated d-spacings of the (001) plane for the SL-SnS2, EL-SnS2, and FL-SnS2 were 0.594, 0.60, and 0.61 nm, respectively, in which the SL-SnS2 showed good agreement with that (0.589 nm) of the hexagonal SnS2 nanostructure. The peak intensities of the SL-SnS2 and EL-SnS2 were higher than that of the FL-SnS2, indicating the good crystallinity of the SL-SnS2 and EL-SnS2. The strong XRD peaks of the SL-SnS2(001) indicate that the 1584
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Table 1. Comparison Table of the Prepared SnS2 Nanostructure Capacitors with the Reported S-Doped Metal Oxide Capacitors materials
specific capacitance
current density
cycle no.
retention
refs
SnS2/MoS2 SnS2 nanosheets m-SnS2 SnS nanorods nano SnS carbon-coated SnS 2D CoSNC CF-SnS2 SnS2 particles EL-SnS2 SL-SnS2 FL-SnS2
105.7 F/g 89.4 F/g 213.2 F/g 70 F/g 14.98 F/g 28.47 F/g 360.1 F g 524.5 F/g 93.8 F/g 390.38 F/g 117.11 F/g 431.82 F/g
2.35 A/g 1 A/g 1 A/g 0.5 mA cm2 1 A/g 200 mA/g 1.5 A/g 0.08 A/g 0.5 A/g 1 A/g 1 A/g 1 A/g
1000 1000 1000 500 1000
94.4% 89% 60 106
2000 1000 500 2000 2000 2000
90% 68 95 ∼82 ∼80 ∼90
18 11 11 14 28 29 30 33 34 this work
tures, the FL-SnS2 exhibited the highest surface area (64.8 m2/ g), whereas the SL-SnS2 and EL-SnS2 showed surface areas of ∼27.4 and 32.9 m2/g, respectively. The pore size of the SnS2 nanostructures (Figure 4c) showed two different levels of pores. The FL-SnS2 and EL-SnS2 nanostructures had a pore diameter less than 10 nm with a maximum diameter of ∼5.1 nm in the case of EL-SnS2 and ∼7.5 nm for the FL-SnS2 sample. In the case of the SL-SnS2, the pore size was larger than 10 nm, indicating that it is macroporous in nature. XPS was conducted to understand the surface electronic states and chemical composition, that is, types of tin (Sn) and sulfur (S) bonds as well as the percentage of Sn and S atoms present in the synthesized sample. The XPS survey spectrum (Figure 4d) provides a complete view on the surface elemental composition of the SnS2 nanostructures. Figure S4 shows the atomic percentage of the elements on the SL-SnS2, EL-SnS2, and FL-SnS2 surface. The SL-SnS2 showed 63.49% tin and 36.51% sulfur; the FL-SnS2 contained 59.89% tin and 41.11% sulfur; and the EL-SnS2 showed 65.76% tin and 34.23% sulfur, as shown in Figure 4e. At a higher resolution, the deconvoluted XPS spectra of the FL-SnS2 displayed two sharp peaks at ∼486.9 and ∼495.3 eV (Figure 4e), which are characteristics of Sn 3d5/2 and Sn 3d3/2, respectively. These two peaks were clearly separated by a splitting energy of ∼8.4 eV, which is the representative value of Sn4+ in SnS2. The high-resolution XPS spectra of S 2p displayed two spin orbital coupling peaks centered at 161.9 and 163.1 eV, which were assigned to S 2p3/2 and S 2p1/2, respectively, in the S2− chemical state (Figure 4f). The binding energies related to Sn4+ and S2− were in good agreement with the literature.27,28 In addition to the XPS measurements, EDX analysis confirmed the presence of sulfur and tin in the FL-SnS2 (Figure S3). 2.1. Electrochemical Properties. Morphological changes also affect the electrochemical performance of the materials. To understand these effects, the electrochemical properties of the EL-SnS2, FL-SnS2, and SL-SnS2 electrodes were analyzed by cyclic voltammetry (CV) and charge−discharge (CD) methods. These analyses are the most prominent device for evaluating the capacitive behavior of the materials. Generally, metal sulfide electrodes in the alkaline electrolytes store charge at the interface of the electrode/electrolyte and in the bulk electrode material. Figure 5 presents the CV curves of the ELSnS2, FL-SnS2, and SL-SnS2 electrodes at a 50 mV/s scan rate over the potential range 0.0−0.50 V. Figure S5 shows the CV curves of the EL-SnS2, FL-SnS2, and SL-SnS2 at various scan rates, ranging from 5 to 50 mV/s. The CV curves of the ELSnS2, FL-SnS2, and SL-SnS2 electrodes were different from the
ideal rectangular curves, which originate from the EDLC and indicate pseudocapacitance characteristics. The CV profiles showed oxidation and reduction peaks, such as the anodic peak (positive current density) and cathodic peak (negative current density). A pair of redox peaks in the CV curves suggests that the charge storage happened because of the redox reaction rather than the EDLC. All of the SnS2 morphologies show the presence of strong cathodic and anodic peaks, which are attributed to the redox reaction; the cathodic peak for all ELSnS2, FL-SnS2, and SL-SnS2 electrodes could be attributed to the electrochemical insertion of K+ in the interlayer of the SnS2 layer structures.11 Among these prepared electrodes, the FLSnS2 electrode possessed a significantly larger enclosed area than those of the EL-SnS2 and SL-SnS2 electrodes. This enhanced performance of the FL-SnS2 electrode might be due to the 3D structure of the FL-SnS2, which provides a larger surface area and better pore size than the EL-SnS2 and SL-SnS2 and can expose more active sites for the intercalation of ions. With increasing scan rate, the redox current increased, resulting in a shift of the anodic peak toward the positive potential, whereas the cathodic peak was shifted to the negative potential. The increase of the current response indicates that the kinetics of the interfacial faradic redox reactions and the rate of electronic or ionic transport are rapid enough at the present scan rate. Even at a high scan rate, a similar shape of the CV curves was maintained, which indicates the good capacitive behavior and reversibility of the samples (Figure S5). The specific capacitance was measured by the charge discharged to highlight the capacitive values of the EL-SnS2, FL-SnS2, and SL-SnS2 electrodes over the potential range 0.00− 0.50 V. Figure 5b shows the CD profiles of the EL-SnS2, FLSnS2, and SL-SnS2 electrodes at a current density of 1 A/g. The FL-SnS2 presented better capacitance performance (∼431.82 F/g) at a current density of 1.0 A/g than the EL-SnS2 (∼390.38 F/g) and SL-SnS2 (∼117.72 F/g), which might be due to its unique 3D flowerlike structure. Figure 5c presents the CD curves of the FL-SnS2, whereas Figure 5d,e shows those of the EL-SnS2 and SL-SnS2 at current densities ranging from 1 to 10 A/g. The specific capacitances of the SL-SnS2 at current densities of 1, 2, 3, 5, 7, 8, and 10 A/g were ∼112.72, ∼104.95, ∼67.76, ∼67.00, ∼44.24, ∼35.36, and 28.8 F/g, respectively, whereas for the EL-SnS2, they were 390.38, 366.24, 247.20, 240.30, 171.52, 121.04, and 100.40 F/g, respectively. Similarly, the specific capacitances of the FL-SnS2 were ∼431.82, ∼398.88, ∼312.18, ∼272.00, ∼230.70, ∼156.88, and ∼126.20 F/g at current densities of 1, 2, 3, 5, 7, 8, and 10 A/g, respectively. In particular, the FL-SnS2 exhibited a higher 1585
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Figure 6. (a) Capacitance retention vs number of CD cycles and (b) last 15 cycles of CD profiles of the FL-SnS2, EL-SnS2, and FL-SnS2.
indicating that a larger number of electrons and electrolyte ions contribute to the charge and discharge processes.5,32 The electrochemical impedance spectroscopy technique was further used to examine the interfacial properties and electrontransfer properties of the as-synthesized SnS2 electrodes. Figure S8 shows the impedance spectra of the SL-SnS2, EL-SnS2, FLSnS2 electrodes, which show a semicircle in the high-frequency region and a sloping straight line in the low-frequency region. The high-frequency area generally represents the series resistance of the equivalent circuit, which is related to the combination of the solution, intrinsic, and contact resistances at the interface of the electrolyte and electrodes.5 The equivalent series resistance values of the SnS2 electrodes with the SL-SnS2, EL-SnS2, FL-SnS2 are ∼0.5, ∼0.6, and 0.5 Ω, respectively. The semicircle in the high-frequency region corresponds to the charge-transfer resistance and constant phase element between the electrode/electrolyte interface.5 From the inset of Figure S8, it can be clearly seen that the high-frequency region semicircle diameter of the SL-SnS2 and EL-SnS2 electrodes are smaller than that of the FL-SnS2 electrode, implying that the flowerlike structure of the SnS2 electrode possesses a favorable and fast charge-transfer behavior as compare to the other two SnS2 electrodes (SL-SnS2 and EL-SnS2). These results confirmed that the flowerlike structure improved the electrochemical performance and cyclic stability of the FL-SnS2 electrode material. Furthermore, the low-frequency region consists of two straight lines, in which the smaller slope can be attributed as Warburg impedance that is associated with the diffusion of electrolyte in the SnS2 and the large slope demonstrates the capacitance nature of the electrode, respectively.35,36 The Nyquist plots of all SnS2 electrodes (Figure S8) explain the pseudocapacitance nature of the electrode.
specific capacitance because of its unique 3D structure, which offers a better pore size and large surface area, which may provide more active sites for the electrolyte intercalation during the CD process and maximize its utilization, as an electrode material.5 Figure 5f presents the specific capacitance plotted as a function of current density for the EL-SnS2, FL-SnS2, and SLSnS2 samples. The specific capacitance decreased with increasing current density owing to the decreased penetration of electrolyte into the pores of the electrode materials.5 The specific capacitance was higher than the previously reported literature value for the SnS2 supercapacitor electrode materials (Table 1). For example, the tin sulfide nanorods showed ∼70 F/g at 0.5 mA/cm2 and molybdenum-doped fewlayered SnS2 showed ∼213.2 F/g at 1 A/g, which is lower than that of the as-prepared FL-SnS2.11,14 The cycling stability is an important issue in relation to the energy storage characteristics of supercapacitors. The cycling stabilities of the EL-SnS2, FLSnS2, and SL-SnS2 electrodes were examined by CD analysis using eq 4. As shown in Figure 6a, the SnS2 nanostructures (EL-SnS2, FL-SnS2, and SL-SnS2) showed good cycling stability at 5 A/g and retained ∼90, ∼82, and ∼80% capacitance for the FL-SnS2, EL-SnS2, and SL-SnS2, respectively, after 2000 CD cycles, showing that the material has better cycling stability than those reported previously for sulfur- and metal oxide-based materials.4,11,14,28−30 Furthermore, at a lower current density of 3 A/g, the FL-SnS2 (Figure S6) retained 70% capacitance with an excellent Coulombic efficiency of 95% at the end of 4500 CD cycles, which further highlights the advantage of the SnS2 electrode. For more insight, the morphology of the FL-SnS2 electrode was also analyzed after 4500 consecutive CD cycles using SEM analysis. In Figure S7, the SEM images proved that the petals of the FL-SnS2 become irregular during the large cycling process but the morphology of the FL-SnS2 was almost similar to that of the fresh FL-SnS2 electrode. This highlights the advantages of the present as-synthesized FL-SnS 2 architecture. The capacitance of the FL-SnS2 was significantly higher than those of the other EL-SnS2 and SL-SnS2materials. On the other hand, for a broader context of the work, the capacitance of the FL-SnS2 was compared with those of the other SnS2-based materials, as listed in Table 1. In addition, the retention of the SnS2 nanostructure electrode materials highlights their excellent long-term cycling stability and suggests the high reversibility and excellent electrochemical stability of the material.5,31 Clearly, the as-synthesized FL-SnS2 exhibited excellent rate capability compared to the EL-SnS2 and SL-SnS2. In the cycling stability test of the EL-SnS2, FL-SnS2, and SL-SnS2, despite showing a similar trend (Figure 6b), they exhibited significantly higher charging and discharging times,
3. CONCLUSIONS Three different SnS2 nanostructures were synthesized using a facile and simple solvothermal method. The sheet-assembled FL-SnS2 achieved a higher specific capacitance of ∼431.82 F/g compared to the SL-SnS2 and EL-SnS2, demonstrating that it is an efficient and effective method to enhance the electrochemical performance of the materials through modifiable morphological structures. Moreover, the FL-SnS2 has a larger surface area and better average pore size, which results in a higher specific capacitance, which perfectly matches the experimental results. SnS2 is suitable for large-scale production because of its facile synthesis method. 1586
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4. EXPERIMENTAL SECTION 4.1. Materials. TAA was obtained from Sigma-Aldrich. Tin chloride pentahydrate was acquired from Alfa Aesar. Acetic acid was purchased from Junsei (99.7%), and ethyl alcohol was provided by Duksan Pure Chemicals Co. Ltd., South Korea. Deionized water was obtained from a Pure Group 30 water purification system. 4.2. Characterization. The surface morphology and internal structure of the as-prepared samples were examined by SEM (Hitachi-S4800-Japan) and field emission transmission electron microscopy (FE-TEM-Technai G2 F20-FEI-USA). The crystallinity and phase were analyzed by XRD (PRO-MPD, PANalytical, The Netherlands) using Cu Kα radiation (λ = 0.15405 nm). The surface composition and chemical behavior of the samples were examined by XPS (ESCALAB-250, Thermo Fisher Scientific, U.K.) using a monochromatized Al Kα X-ray source (hν = 1486.6 eV). The textural properties were measured by the N2 adsorption−desorption method using a volumetric gas adsorption apparatus (ASAP-2020, Micromeritics Inc., USA). The pore size distribution was estimated using the Barrett−Joyner−Halenda model. 4.3. Electrochemical Measurements. The electrochemical performance was examined using a three-electrode system with a platinum sheet, AgCl/Ag electrode, and Ni foam coating with the mixture of the electrode material as the counter, reference, and working electrodes, respectively. The working electrode was prepared using the electrode material, activated carbon, and Nafion solution at a mass ratio of 80:10:10, and 1 mL of ethanol was then added and stirred for 20 min. Subsequently, the mixed and ground slurry was coated on the Ni foam current collector and dried at 80 °C for 12 h. All electrochemical measurements were carried out using a VersaSTAT 3, Princeton Research, USA Electrochemical Workstation, in a 2 M KOH aqueous solution. The loading of the active electrode material was calculated based on the mass difference before and after drying, and the mass of each electrode material was ∼1.5 mg. Galvanostatic CD and CV are generally used to calculate the electrochemical performance of the electrode materials. The specific capacitance can be calculated as follows:11,19 C = I Δt /mΔV
further characterization. The sheetlike SnS2 synthesized using acetone as the solvent is abbreviated as SL-SnS2, flowerlike SnS2 synthesized using methanol as the solvent is abbreviated as FLSnS2, and ellipsoid-like SnS2 synthesized using water as the solvent is abbreviated as EL-SnS2.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01939. Different precursors, solvents, and reaction conditions used to synthesize SnS2 nanostructures to enhance surface area; SEM, TEM, and EDX images of SnS2 nanostructures; specific surface area, pore diameter, and pore volume of the SnS2 nanostructures synthesized by the solvothermal method using different solvents; percentage of elements in SnS2 nanostructures; cyclic voltammograms of the EL-SnS2, FL-SnS2, and SL-SnS2 at various scan rates; cyclic stability and corresponding Coulombic efficiency of the FL-SnS2; SEM images of the FL-SnS2 electrode after 4500 CD cycles; and Nyquist plots of the EL-SnS2, SL-SnS2, and FL-SnS2 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected],
[email protected]. Phone: +82-53-810-2517. Fax: +82-53-810-4631 (S.A.A.). *E-mail:
[email protected] (M.H.C.). ORCID
Ziyauddin Khan: 0000-0002-8242-7547 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by Priority Research Centers Program (grant no: 2014R1A6A1031189) through the National Research Foundation of Korea funded by the Ministry of Education in South Korea.
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where I is the constant discharge current and t is discharge time. The potential window is symbolized by ΔV, and the mass of the active electrode material is represented by m. 4.4. Synthesis of SnS2 Nanostructures. Table S1 lists the various reports of hydrothermal/solvothermal SnS2 nanostructures with different morphologies. The SL-SnS2, FL-SnS2, and EL-SnS2 were synthesized using a modified solvothermal method that involved changing the solvent during the reaction procedure to confirm the effects of the solvent on the morphology of the material. In a typical procedure, an appropriate amount of tin chloride pentahydrate was dissolved in an acetic acid solution and stirred magnetically for 5 min, which was followed by the addition of 55 mL of solvent (methanol, acetone, and water) to the solution and then 1.5 g of TAA as the sulfur source. After dissolving the precursor, the solution was transferred to a Teflon-lined stainless steel autoclave and heated in an electric oven at 160 °C for 10 h. After the reaction, the autoclave was allowed to cool naturally to room temperature and the yellow-colored sample was collected after vacuum filtration. The final product was dried in a hot air oven at 80 °C for 6 h and stored in a desiccator for 1587
DOI: 10.1021/acsomega.7b01939 ACS Omega 2018, 3, 1581−1588
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